A lithium ion secondary battery is a small-sized and lightweight chargeable battery; is storage capacity per volume unit and weight unit is large; and is widely utilized in mobile phones, laptop personal computers, personal digital assistants, video cameras, digital cameras and the like, and is indispensable in each small-sized and lightweight mobile device having relatively large power consumption. From such characteristics, it is considered that the lithium ion secondary battery becomes key technology in storage battery technology as energy conservation and energy storage. Energy conservation includes application to vehicle-mounted uses such as electric vehicles (EV) and hybrid electric vehicles (HEV), and energy storage includes application to a fixed power source for efficient use of wind power generation, solar power generation and night power. In practical use of those uses, further high performance, high capacity and low cost are desired. On the other hand, in recent years, accident and recovery of lithium ion secondary battery occur one after another, and safety is emphasized. Thus, high reliability in the battery is strongly desired.
The most widespread cathode material of the lithium ion secondary battery is lithium cobaltate. The lithium cobaltate is a material having excellent performance, and is therefore used in many consumer equipments. However, since it is a rare metal, there are the problems such that it is expensive; change in the price is large; and safety is low. Other materials include lithium nickelate and lithium manganate. Although the lithium nickelate has high capacity and excellent high temperature cycle, it is poor in safety. Although the lithium manganate has the characteristics that safety such as overchargeability is excellent, and the price is low, capacity is low and high temperature cycle is inferior. Furthermore, a nickel-manganese-cobalt material is developed. This material can reduce cobalt, resulting in low cost, and safety is improved. For this reason, practical use of the material is progressing.
Under the above situation, olivine-type lithium oxides are low in environmental load and are rich as resources, and are therefore considered to be low cost material. Furthermore, the olivine-type lithium oxides have high capacity and excellent thermal stability at charging, and therefore can improve safety even in the emergency such as overcharge. Therefore, the olivine-type lithium oxides are expected as a cathode material (Patent Documents 1 to 3).
On the other hand, charging time of the lithium ion secondary battery is from about 1 to 5 hours, and very long time is required. For this reason, the demand to quick charge is increasing. Quick charge enables downsizing of equipments used, and can increase use frequency per unit time. This makes it possible to cut down an auxiliary battery and to decrease battery cost. To enable quick charge, Patent Document 4 proposes that an A/C ratio which is the ratio of an anode capacity A to a cathode capacity C is adjusted to from 1.1 to 1.6. In this invention, although quick charge characteristics are excellent, but quick charge/discharge cycle life is not clearly described.
In the case of conducting quick charge, large charging current must be flown through a lithium ion secondary battery, and speed of lithium ions inserted in an anode retards to charging rate. As a result, electrons on the anode react with lithium ions, and metallic lithium is precipitated on the anode. This greatly decreases battery performance and adversely affects safety. For this reason, quick charge in a short period of time of 15 minutes or less could not have conventionally been conducted.    Patent Document 1: JP-A 2002-216770    Patent Document 2: JP-A 2002-117902    Patent Document 3: JP-A 2002-117907    Patent Document 4: JP-A 2008-171661